It's quite the opposite. No bleed over increases left to right focus. The closeness of the drivers to the ear forces a solid centre where with speakers, you have to be in the sweet spot for staging to work. And with open headphones the left does hear the right and so forth. Not as loudly as speakers, but hears it none the less. But the hard pan of sound makes the stage go through the head. The more open the design the more it's pulled out of the head ala HD800. And in the other direction, IEMs have a solid centre but hard left/right panning through the inside of the head. Shoulder checks happen quite a bit with my HD800 but never with the UM3X.
As for the Mjolnir, it staged very very well.